Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. As used herein, the term “low IF” refers to both baseband and the intermediate frequency signals. A filtering stage filters the low IF signals to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard.
More specifically, the one or more intermediate frequency stages that mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency signal often utilize CMOS-based mixer designs. One problem that is known for CMOS-based mixers, however, is that flicker noise (1/ƒ) appears at the output of the mixer even though the output loads are free of flicker noise. In general, flicker noise significantly increases in magnitude as a frequency approaches 0 Hz (DC). For some communication systems, the effects of flicker noise are insignificant because the systems operate at higher frequencies. For other communication systems, for example, GSM systems, flicker noise is significant. In some cases, the flicker noise is sufficiently significant that particular mixer designs cannot be readily used.
Active mixers typically comprise input transconductance elements, switches and an output load. Many of these components are formed with noisy transistors. The loads, as well as the transconductance elements (field effect transistors), generate both flicker noise and white noise. MOSFET devices also generate noise during switching. As a differential pair of a mixer switches from one side to the other side, for example, a skew in the switching instant modulates a differential current waveform at the mixer output. This skew retards the timing of the zero-crossing by the local oscillation differential voltage. Thus, a low frequency noise at the gate of the input switch appears at the output without frequency translation and therefore corrupts a signal that is down converted from RF to baseband or to a low IF signal. What is needed, therefore, is a mixer for use in CMOS radio transceiver circuits that reduces or eliminates the effects of flicker noise and, more generally, noise from switching, especially for transceivers that operate at low frequencies.